Method and arrangement for providing Vias in printed circuit boards

ABSTRACT

A method of providing thermal vias in a printed circuit board that includes one or more layers of board material is disclosed. The vias provide for conducting heat from components mounted on the board. One or more holes ( 4 ) are provided in a printed circuit board that may include several metal layers. A metal ball ( 6 ) is inserted into each hole and subjected to pressure such as to deform said ball, and tightly fixating the resultant slug against the wall ( 5 ) of said hole. The deformed ball or slug fixed in the hole, which may have a metallised inner surface, functions to conduct heat and/or electricity between a metallised topside ( 2 ) and bottom side ( 3 ) of the printed circuit board and also between intermediate metallised layers in the case of a multi-layer board.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of copending InternationalApplication No. PCT/SE01/02856 filed Dec. 19, 2001, and claiming apriority date of Dec. 29, 2000, which designates the United States.

TECHNICAL FIELD

The present invention relates to a method of providing vias in a printedcircuit board, and also to a printed circuit board in which vias arearranged in accordance with the method.

BACKGROUND OF THE INVENTION

When surface-soldering power components to a printed circuit board, suchas RF power transistors for instance, it is necessary to improve thethermal conductivity of the board with the use of thermal vias. This isbecause the actual board material, which is often comprised of some kindof polymeric material, has very poor thermal conductivity, whereasmetallic material, such as copper, is a very good conductor of heat.

At present, thermal vias are formed by drilling one or more holes in theboard material, on the surface to which components are to be soldered.In the following stages of board manufacture, the inner surfaces ofrespective vias are coated with a thin layer of metallic material,preferably copper. It is essentially this thin copper layer on the innersurface of the holes that conducts heal through the board.

One problem with the present-day production of thermal vias is that thesolder used to fasten components to the board penetrates into the holes,therewith diminishing the amount of solder that is effective in securingsaid components. In addition, solder droplets are formed on the oppositeside of the board in relation to said components. These solder dropletsare problematic, since they greatly impair the thermal contact betweenall underside of the board and a cooler, and must therefore be removedat a later stage in some suitable way.

SUMMARY OF THE INVENTION

One object of the present invention is to lighten this problem and toprovide improved cooling.

According to a first aspect of the present invention, this object can beachieved with a method of providing vias in printed circuit boards orlike structures that comprise one or more layers of board material,comprising the steps of:

-   -   forming holes in the layer or layers of board material;    -   inserting a ball into each hole that is intended to serve as a        conductor between different conductive planes on or in the        layers of board material;    -   pressing each ball into a respective hole firmly such as to        cause deformation of the ball, such that the outer delimiting        surface of the defamed ball will lie in abutment with and in        contact with the inner delimiting surface of said hole and its        various conductive planes or the inner conductive delimiting        surface of said hole.

The ball may serve as a thermal conductor and/or as an earth.

The object can also be achieved by a method of providing vias in printedcircuit boards, comprising the steps of:

-   -   providing a board material;    -   drilling at least one hole in said board;    -   inserting a ball into said hole; and    -   pressing said ball firmly into said hole, so as to form sand        copper vias.

The method may further comprise the step of metallizing the holes and/orthe step of plating the topside with copper. The method may furthercomprise the step of plating the bottom side with copper and/or the stepof etching a pattern into said copper layer. The step of providing aboard may comprise the steps of:

-   -   providing a first board layer and a second board layer, wherein        said boards each comprise a pattern etched layer;    -   laminating said two boards with a pre-preg.

According to a second aspect of the present invention, said object canbe achieved with a printed circuit board comprising one or more layersof board material provided with conductor patterns and one or more vias,wherein

-   -   the printed circuit board has disposed therein one or more holes        which extend through said layer or layers;    -   each hole has provided therein an electrically and/or thermally        conductive and deformed ball which has been deformed in said        hole subsequent to having been inserted and pressed firmly        therein, wherewith said deformed balls seal against and are        fixated by essentially the inner delimiting surface of said        hole.

The balls can be made of copper. The printed circuit board may furthercomprise a metal layer on the topside and/or the bottom side of saidboard, said layer covering the ends of the deformed balls.

One advantage afforded by the present invention is that the printedcircuit boards have a much lower thermal resistance from one upper sideto an underside of said boards in comparison with boards manufactured inaccordance with the present standpoint of techniques.

Another advantage afforded by the present invention is that probing ofthe vias is made easier when they are fully closed.

A further advantage afforded by the present invention is that it allowsthe power components to be surface-mounted at the same time as all othercomponents are surface-mounted on the board, therewith greatly reducingcosts in comparison with traditional mounting processes.

Yet another advantage of the present invention is that a cooler orchassis need not fulfil the same high requirements with respect tosurface flatness, since components, such as RF power transistors, can besurface-mounted directly on the printed circuit board by solderingprocesses.

The invention will now be described in more detail with reference topreferred embodiments and also with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C illustrate the insertion of a copper ball into a hole in aprinted circuit board and the deformation of said ball under pressure,in accordance with the invention.

FIGS. 2A-E illustrate the insertion of several copper balls into holesin a printed circuit board and the deformation of said balls underpressure to form vias in accordance with the invention, and also showplating and etching of patterns on the printed circuit board.

FIGS. 3A-F illustrate the insertion of a copper ball in a hole in amulti-layer printed circuit board and the deformation of said ball underpressure to form a via in accordance with the invention, and also showplating and etching of patterns in the top layers.

FIGS. 4A-C illustrate the insertion of an elongate copper body in anarrow hole in a. printed circuit board and the deformation of said bodyunder pressure, such as to form a via in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A is a cross-sectional view of a board 1 for the manufacture of aprinted circuit board. The board 1 comprises board material 10 thatincludes a metallised topside 2 and a metallised bottom side 3. Thesemetallised sides form a basis for conductor patterns. Conductor patternsare etched from the electrically conductive topside and bottom side inaccordance with methods which are well known to the person skilled inthis technical field and which therefore need not be described in moredetail here. The board material may be any suitable commerciallyavailable material, such as FR4 or BT. The board includes a hole 4 thathas a metallised outer surface 5 which is in contact with the metallisedtopside and bottom side of said board.

FIG. 1B is a cross-sectional view of the board 1, showing a metal ball6, e.g. a copper ball, inserted in the hole 4. Copper balls can beinserted into holes in printed circuit boards more easily thancylindrical copper bodies and can be made in all conceivable sizes, witha tolerance of +/−20 micrometers.

FIG. 1C shows the hole 4 practically filled completely as a result ofdeforming the copper ball 6 in said hole under pressure wherewithmetallic contact is achieved between the outer surface of the deformedball and the metallised outer surface 5 of the hole 4. The deformedcopper ball in the metallised hole enables heat, and possiblyelectricity, to be conducted from the metallised topside 2 to themetallised bottom side 3 of the board.

FIG. 2A illustrates part of a non-perforated or imperforate boardmaterial 7.

FIG. 2B shows the board material of FIG. 2A in which three holes 8 havebeen drilled. These holes may have been metallised.

FIG. 2C shows how balls 9 have been inserted into respective holes andpressed firmly and deformed therein, so as to form sand copper vias.

FIG. 2D illustrates a copper plated topside 11 and bottom side 12 ofsaid board material, wherewith the end surfaces of the solid coppervias, i.e. the deformed balls 9, have also been covered by the copperlayer.

FIG. 2E illustrates patterns 13 etched on the topside of the boardmaterial 7.

FIG. 3A is a cross-sectional view of part of two board layers 14 and 15,said layers having been pattern-etched separately.

FIG. 3B shows how the two board layers 14, 15 have been laminated with apre-preg 16.

FIG. 3C shows via holes 17 and cooling holes 18 that have been drilledthrough the composite structure 14-16, FIG. 3D shows the drilled holesprovided with metallised inwardly lying layers 19 and 20, which may beof the so-called copper coating type.

FIG. 3E illustrates a solid copper via in the multi-layer printedcircuit board, said via having been formed by inserting a copper ball 21into said hole and subjecting the ball to pressure to form a plug/via.

FIG. 3F shows the printed circuit board plated with copper, wherein acopper layer 22 and 23 also covers the end surfaces of the plug/coppervia. Patterns can then be etched in the top layers on the topside andthe bottom side of the board in accordance with conventional methods.

FIG. 4A is a cross-sectional view of an alternative embodiment of aprinted circuit board 1 that includes a metallised top side 2; ametallised bottom side 3 and a hole 25 which is narrow in relation toboard thickness such narrow holes possibly being necessary in certainvia applications.

FIG. 4B shows the board of FIG. 4A with an elongated ball 26 inserted inthe hole 25; it is necessary to stretch the ball in order for the ballto :6.t in the narrow hole and fill-out the hole cavity with anoptionally metallised inner surface. Generally speaking, it is alwaysnecessary to adapt the ball diameter to the hole diameter and to theboard thickness.

FIG. 4C shows how the elongated ball 26 has been subjected to pressureand deformed in the hole such as to essentially fill the entire holecavity.

The metallic plugs are formed by deformed metallic balls pressed firmlyinto holes in the board material, so as to :fill the hole cavitiespractically completely. The balls will fill the hole in a directionorthogonal to the longitudinal direction of the holes and, as they arepressurized, expand into contact with practically the whole of the innerwall of said hole, therewith providing good metallic contact between theresultant solid copper vias and the existing metallised hole walls.

Although the illustrated embodiments of the printed circuit boardsprovided with vias are of a single-layer type and a multi-layer typethat includes two board layers, it will be understood that the presentinvention can also be applied with boards that have more layers thantwo, wherewith the copper plugs are able to function as thermal viasbetween a topside and a bottom side of the board and between thedifferent metallic layers, and can also function as earth connectionsbetween the different metallic layers per se.

It will also be understood that the invention is not restricted to theaforedescribed and illustrated exemplifying embodiments thereof and thatmodifications can be made within the scope of the accompanying claims.

1. A method of providing vias in printed circuit boards or likestructures that comprise one or more layers of board material,comprising the steps of: forming holes in the layer or layers of boardmaterial; inserting a ball into each hole that is intended to serve as aconductor between different conductive planes on or in the layers ofboard material; pressing each ball into a respective hole firmly such asto cause deformation of the ball, such that the outer delimiting surfaceof the defamed ball will lie in abutment with and in contact with theinner delimiting surface of said hole and its various conductive planesor the inner conductive delimiting surface of said hole.
 2. The methodaccording to claim 1, wherein the ball serves as a thermal conductor. 3.The method according to claim 1, wherein the ball serves as an earth. 4.A printed circuit board comprising one or more layers of board materialprovided with conductor patterns and one or more vias, wherein theprinted circuit board has disposed therein one or more holes whichextend through said layer or layers; each hole has provided therein anelectrically and/or thermally conductive and deformed ball which hasbeen deformed in said hole subsequent to having been inserted andpressed firmly therein, wherewith said deformed balls seal against andare fixated by essentially the inner delimiting surface of said hole. 5.A printed circuit board according to claim 4, wherein the balls are madeof copper.
 6. A printed circuit board according to claim 4, furthercomprising a metal layer on the topside of said board, said layercovering the ends of the deformed balls.
 7. A printed circuit boardaccording to claim 4, further comprising a metal layer on the bottomside of said board, said layer covering the ends of the deformed balls.8. A printed circuit board according to claim 4, further comprising ametal layer on the topside and bottom side of said board, said layercovering the ends of the deformed balls.
 9. A method of providing viasin printed circuit boards, comprising the steps of: providing a boardmaterial; drilling at least one hole in said board; inserting a ballinto said hole; and pressing said ball firmly into said hole, so as toform sand copper vias.
 10. The method according to claim 9, furthercomprising the step of metallizing the holes.
 11. The method accordingto claim 9, further comprising the step of plating the topside withcopper.
 12. The method according to claim 9, further comprising the stepof plating the bottom side with copper.
 13. The method according toclaim 11, further comprising the step of etching a pattern into saidcopper layer.
 14. The method according to claim 9, wherein the step ofproviding a board comprises the steps of: providing a first board layerand a second board layer, wherein said boards each comprise a patternetched layer; laminating said two boards with a pre-preg.